The present invention relates to a biaxially oriented laminate polyester film for a magnetic recording medium. More specifically, it relates to a biaxially oriented laminate polyester film useful as a base film for a magnetic recording medium which is excellent in running durability and electromagnetic conversion characteristics and is almost free from drop-out. Further, the invention relates to a biaxially oriented laminate polyester film excellent in durability against deterioration under dry heat and controllability of precipitation of foreign matters.
In recent years, progress has been remarkably made toward higher-density magnetic recording media, as exemplified by the development and practical use of a thin metal film magnetic recording medium having a thin ferromagnetic metal film formed on a non-magnetic base film by a physical deposition method such as vacuum deposition, sputtering or the like or a plating method, and a thin layer coated magnetic recording medium having a layer having a thickness of 2 xcexcm or less, the layer being formed from a needle-like magnetic powder such as a metal powder or an iron oxide powder.
As examples of the thin metal film magnetic recording medium, JP-A 54-147010 discloses a magnetic recording medium having a first thin Co-film magnetic layer coated on a non-magnetic base film and a second thin Co-film magnetic layer which has a larger thickness than the first thin Co-film magnetic layer and is formed on the first thin Co-film magnetic layer via a non-magnetic material layer. Further, JP-A 52-134706 discloses a vertical magnetic recording medium formed from a Coxe2x80x94Cr alloy.
As an example of the thin layer coated magnetic recording medium, xe2x80x9cTechnical Report MR 94-78xe2x80x9d issued by the Institute of Electronics and Communication Engineers of Japan (1995-02) discloses a high-density magnetic recording on the basis of an extremely thin layer coated magnetic recording medium.
The conventional coated magnetic recording media (magnetic recording media produced by incorporating a magnetic powder in an organic polymer binder and coating the mixture on a non-magnetic base film) has a low recording density and uses a long recording wavelength, so that the thickness of their magnetic layer is as thick as about 2 xcexcm or more. In contrast, a thin ferromagnetic metal film formed by thin film-forming means such as vapor deposition, sputtering or ion plating has a remarkably small thickness of as small as 0.2 xcexcm or less. Further, concerning the extremely thin-layer coated media, some media having a thickness of 0.13 xcexcm are provided and hence, have a very small thickness, although they have a non-magnetic underlying layer.
In the above high-density magnetic recording media, therefore, the surface condition of the non-magnetic base film has a great influence on the surface characteristics of the magnetic layer. In the thin metal film magnetic recording medium in particular, the surface condition of the non-magnetic base film develops directly into an unevenness of surface of a magnetic layer (magnetic recording layer).
Further, the thin metal film magnetic recording medium involves the running property of the surface of a thin metal film as one of serious problems in actual use. In the coated magnetic recording medium having a base film coated with a mixture of a magnetic powder incorporated in an organic polymer binder, the running property of the magnetic layer surface can be improved by dispersing a lubricant in the binder.
In the thin metal film magnetic recording medium, however, such measure cannot be taken, and it is very difficult to maintain the stable running property. Particularly, it has a problem that its running property is inferior under high-temperature and high-humidity conditions. Further, in this case, there is another drawback that a reduction in the output during its repeated use is larger than that of a coated magnetic recording medium.
On the other hand, from the viewpoint of the formation of a non-magnetic base film and handling properties such as transportation, winding-up and unwinding in processing steps, an extreme smoothness of the film surface causes slipperiness between films to be poor and causes a blocking phenomenon to take place, so that the roll formation thereof is deteriorated and that the yield of products decreases. As a consequence, the production cost thereof increases. In view of a production cost, therefore, the surface of the non-magnetic base film is as rough as possible.
As described above, the surface of the non-magnetic base film is required to be smooth from the viewpoint of electromagnetic conversion characteristics, whereas it is required to be rough from the viewpoint of handling properties and film costs.
For the production of a high-density magnetic recording medium of excellent qualities, it is required to satisfy the above two contradictory requirements at the same time.
JP-A 5-194772 discloses a polyester film for a magnetic recording medium, the polyester film having one surface coated with a primer layer of a continuous thin film for a magnetic layer, wherein the continuous thin film surface of the primer layer has (A) small protrusions which have a height of 13 nm or less and comprise particles having an average particle diameter of less than 0.06 xcexcm as cores, (B) large protrusions which have a height of 30 nm or less and comprise particles having an average particle diameter of 0.06 xcexcm or more as cores, and (C) fine protrusions formed solely of a resin forming the primer layer, the numbers of these protrusions satisfying the following expressions:
ANxe2x89xa71.0xc3x97106 (pieces/mm2)
BNxe2x89xa71.05xc3x97104 (pieces/mm2)
ANxe2x89xa6xe2x88x923.4xc3x97102xc3x97BN+13.6xc3x97106 (pieces/mm2)
CNxe2x89xa64.0xc3x97106 (pieces/mm2)
in which AN is the number (pieces/mm2) of the small protrusions, BN is the number (pieces/mm2) of the large protrusions and CN is the number (pieces/mm2) of the fine protrusions,
the continuous thin film formed solely of a resin forming the primer layer has a fine surface roughness, RaS, of 1.10 nm or less, and the continuous thin film has a surface roughness, Ra, of 1 to 10 nm.
JP-A 5-298670 discloses a polyester film for magnetic recording medium, the polyester film having one surface coated with a primer layer of a continuous thin film for a magnetic layer, wherein the continuous thin film surface of the primer layer has (A) small protrusions which have a height of 13 nm or less and comprise particles having an average particle diameter of less than 0.06 xcexcm as cores, (B) large protrusions which have a height of 30 nm or less and comprise particles having an average particle diameter of 0.06 xcexcm or more as cores, and (C) fine protrusions formed solely of a resin forming the primer layer and having the longest major diameter of 0.30 xcexcm or less, the numbers of these protrusions satisfying the following expressions:
ANxe2x89xa71.0xc3x97106 (pieces/mm2)
BNxe2x89xa71.05xc3x97104 (pieces/mm2)
ANxe2x89xa6xe2x88x923.4xc3x97102xc3x97BN+13.6xc3x97106 (pieces/mm2)
1.0xc3x9710 (pieces/mm2)xe2x89xa6CNxe2x89xa61.0xc3x97104 (pieces/mm2)
in which AN is the number (pieces/mm2)of the small protrusions, BN is the number (pieces/mm2) of the large protrusions and CN is the number (pieces/mm2) of the fine protrusions,
the continuous thin film formed solely of a resin forming the primer layer has a fine surface roughness, RaS, of 1.10 nm or less, the continuous thin film has a surface roughness, Ra, of 1 to 10 nm, and when the polyester film is continuously heated in air at 160xc2x0 C. for 5 minutes, the continuous thin film can be controlled to show that the rate of deposition of polyester oligomer fine crystals on the film surface is 0.8% or less.
The above polyester film can attain a smoother base film on the magnetic layer surface side to some extent. However, particles contained in the base film on the magnetic layer surface side for improving the running durability perform insufficient dispersion, so that extra-large protrusions increase in number, and there is therefore a problem that drop-out is caused or that a magnetic head is caused to suffer an partial abrasion to invite a reduction of output.
In the high-density magnetic recording medium, the surface condition of the non-magnetic base film exerts a great influence on the surface characteristics of a magnetic layer as described already, and studies are therefore under way on measures to control coarse protrusions on the surface of the base film, from the standpoint of polyester production as well.
One of the causes of formation of protrusions on the film surface is that a catalyst added during the production of the polyester, an antimony compound in particular, precipitates in the polyester. For example, the antimony compound accomplishes a high polymerization rate and has advantages that an obtained polyester is excellent in various properties such as heat stability, terminal carboxyl group amount, softening point, and the like. However, it has a drawback that it produces a precipitate in the polyester as described above.
As catalysts for polymerization of a polyester, in addition to the above antimony compound, JP-B 47-15703, JP-B 47-16193, JP-B 47-42756, etc. disclose a germanium compound, and JP-A 48-31293 and JP-A 52-33996 disclose use of a titanium compound. When a germanium compound is used, no precipitate is formed unlike the use of an antimony compound. Since, however, the germanium compound causes a large side reaction during the polymerization, an obtained polyester has a low softening point, and when it is made into a film, the film comes to have a low mechanical strength. When a titanium compound is used, the polymerization rate is very high, and no precipitate is formed unlike the use of an antimony compound. However, it involves a problem that an obtained polyester is poor in heat stability.
Meanwhile, it is general practice to improve a polyester in heat stability by adding a phosphorus compound in the production of the polyester. However, the phosphorus compound reacts with most polymerization catalysts to decrease the catalyst activity or to cause a precipitate. Particularly, the titanium compound is deactivated to a considerable degree in the presence of a phosphorus compound.
Further, in the production of a base film for a deposited video film, it is difficult to secure running properties and wind-up properties of the film since the surface of the film is required to have an ultra-flatness. Under the circumstances, it is general practice to smoothen the film surface (e.g., formation of a smooth layer). For this purpose, it is required in many cases to set a heat-setting temperature during the film formation at a little higher temperature (230 to 240xc2x0 C.) than an ordinary heat-setting temperature (205 to 220xc2x0 C.). However, it has been found that, when a film undergoes stopping of running inside a heat-setting machine (stenter) due to breakage of the film, etc., a thin polyester film comes to be treated under dry heat at a high temperature, so that the film deteriorates in a surprisingly short period of time to become fine powder and adhere to the inside surface of the stenter, thereby causing a trouble that the powder constitutes foreign matters during the subsequent film formation. The above trouble is not caused when an antimony compound is used as a catalyst, but it is caused when a titanium compound or a germanium compound is used. The above trouble is strikingly caused when a titanium compound in particular is used.
Concerning the production of a polyester, JP-A 6-340734 discloses that a polyester having an excellent surface flatness as a film can be produced by using a calcium compound, a magnesium compound, a phosphorus compound and an antimony compound as a catalyst and a stabilizer and bringing their contents and amount ratio into specific ranges.
Further, JP-A 7-48439 discloses that a polyester having, as a film, excellent durability against deterioration under dry heat and excellent surface flatness can be produced by using a calcium compound, a magnesium compound, a phosphorus compound and a titanium compound as a catalyst and a stabilizer and bringing their contents and amount ratio into specific ranges.
Further, JP-A 8-188704 discloses that a polyester having, as a film, excellent durability against deterioration under dry heat and excellent controllability against precipitation of antimony can be produced by using a titanium compound, a germanium compound, an antimony compound and a phosphorus compound as a catalyst and a stabilizer and bringing their contents and amount ratio into specific ranges.
When one of the above processes for the production of a polyester is used and, further, when an obtained polyester is used, for producing a polyester film having a primer layer coated thereon, the formation of a smoother base film on the magnetic surface side can be accomplished to some extent. In recent years, however, it is demanded to achieve a higher order of the recording density, and there is caused a problem that film-surface protrusions which have not so far caused a problem cause drop-out, and the like.
It is a first object of the present invention to provide a biaxially oriented laminate polyester film useful as a base film for a high-density magnetic recording medium that is excellent in running durability and electromagnetic conversion characteristics and causes extremely fewer drop-outs.
It is a second object of the present invention to provide a biaxially oriented polyester film useful as a base film for a high-density magnetic recording medium, which film is excellent in durability against deterioration under dry heat and controllability against the precipitation of foreign matters.
According to studies made by the present inventors, it has been found that the above objects of the present invention are achieved by a biaxially oriented laminate polyester film for magnetic recording medium, which is a laminate film comprising a base layer (A) formed of a polyester resin and a coating layer (B) formed on one surface of the base layer (A), the base layer (A) and the coating layer (B) fulfilling the following requirements (1) to (3):
(1) the base layer (A) substantially contains no fine particles added or contains 0.001 to 0.1% by weight of added fine particles having an average particle diameter of 0.005 to 0.3 xcexcm,
(2) the coating layer (B) contains a binder resin, added fine particles and a surfactant, the added fine particles having an average particle diameter of 10 to 50 nm and being contained in an amount of 0.5 to 30% by weight, and the coating layer (B) having a thickness of 3 to 40 nm, and
(3) the coating layer (B) has a surface which satisfies the following surface characteristics (a) to (d):
(a) the surface has 80,000 pieces/mm2 or less of protrusions having a major diameter of 0.05 xcexcm or more when measured on the basis of a surface photograph taken and enlarged at a magnification of 5,000 with a scanning electron microscope,
(b) the surface has 1 to 40 pieces/xcexcm2 of protrusions, measured on the basis of a surface photograph taken and enlarged at a magnification of 35,000 with a scanning electron microscope,
(c) the surface has 20 pieces/cm2 or less of extra-large protrusions having a major diameter of 10 xcexcm or more, and
(d) the surface has a central plane average roughness (SRa) of 5 nm or less.
The biaxially oriented laminate polyester film of the present invention will be explained further in details.
In the laminate polyester film of the present invention, the polyester resin for forming the base layer (A) refers to an aromatic polyester resin generally known as a base film. Specifically, it is a linear polyester comprising terephthalic acid or naphthalenedicarboxylic acid as a main acid component and ethylene glycol as a main glycol component and having film formability. The above polyester may be any one of a homopolyester and a copolyester. As a polyester, particularly preferred is a polyester which comprises at least 80 mol %, preferably at least 90 mol %, based on the total of recurring units, of an ethylene terephthalate unit or an ethylene-2,6-naphthalate unit.
In the copolyester, the comonomer includes other diol components such as diethylene glycol, propylene glycol, tetramethylene glycol, hexamethylene glycol, neopentyl glycol, polyethylene glycol, 1,4-cyclohexanedimethanol and p-xylylene glycol; other dicarboxylic acid components such as adipic acid, sebacic acid, phthalic acid, isophthalic acid and 5-sodium sulfoisophthalic acid; oxycarboxylic acid components such as p-oxyethoxybenzoic acid; and the like. The amount of the above comonomer is preferably 20 mol % or less, more preferably 10 mol % or less, based on the total amount of the acid components.
Further, a polyfunctional compound having 3 or more functional groups, such as trimellitic acid or pyromellitic acid, may be copolymerized. In this case, it can be copolymerized in such an amount that the polymer is substantially linear, for example, in an amount of 2 mol % or less.
In the laminate polyester film of the present invention, the polyester resin for forming the base layer (A) may be a resin that substantially contains no added fine particles. The term xe2x80x9cadded fine particlesxe2x80x9d denotes fine particles that are intentionally added to the polyester resin or added during the production thereof, and the term xe2x80x9csubstantially contains no particlesxe2x80x9d means that 0.001% by weight or more of fine particles are not contained in the resin. Therefore, particles which are generated from a catalyst or a stabilizer during the production of a polyester resin do not come under the category of the added fine particles.
It is desirable that the polyester resin for forming the base layer (A) should substantially not contain added fine particles as described above. However, the polyester resin may contain a certain amount of fine particles having a specific size depending upon a purpose. When added fine particles are incorporated into the polyester resin for the base layer (A), the average particle diameter of the added fine particles is 0.005 to 0.3 xcexcm, preferably 0.01 to 0.2 xcexcm, particularly preferably 0.02 to 0.2 xcexcm, and the content thereof is 0.001 to 0.1% by weight, preferably 0.005 to 0.08% by weight. When the above fine particles are contained in the polyester of the base layer (A), the running durability of a laminate polyester is improved. When the average particle diameter of the added fine particles exceeds 0.3 xcexcm, undesirably, the polyester film is poor in electromagnetic conversion characteristics.
In the fine particles contained in the polyester resin of the base layer (A), advantageously, the volume shape factor (f) thereof is in a range of 0.1 to xcfx80/6, preferably 0.4 to xcfx80/6. The above volume shape factor (f) is defined by the following equation.
f=V/R3
wherein f is a volume shape factor, V is a volume (xcexcm3) of a particle and R is an average particle diameter (xcexcm) of the particle.
The shape of a particle having a volume shape factor (f) of xcfx80/6 is a sphere (true sphere), and the shape of particles having the volume shape factor (f) of 0.4 to xcfx80/6 includes substantially the shape of a sphere (true sphere) to the shape of an elliptic ball like an egg shape. When particles having a volume shape factor (f) of less than 0.1, e.g., flake-shaped particles, are used, it is difficult to attain sufficient running durability.
Further, the relative standard deviation of the particle diameter of the fine particles, defined by the following equation, is preferably 0.5 or less, more preferably 0.3 or less, particularly preferably 0.12 or less. When the relative standard deviation is larger than 0.5, undesirably, the resulting film has insufficient protrusion uniformity.       Relative    ⁢          xe2x80x83        ⁢    standard    ⁢          xe2x80x83        ⁢    deviation    =                                          ∑                          i              =              1                        n                    ⁢                      xe2x80x83                    ⁢                                    (                              Di                -                                  D                  _                                            )                        2                              n        /          D      _      
wherein Di: diameter (xcexcm) of a circle equivalent to an area of each particle
{overscore (D)}: average value of diameters of circles equivalent to areas       (          =                                    ∑                          i              =              1                        n                    ⁢                      xe2x80x83                    ⁢          Di                n              )    ⁢      xe2x80x83    ⁢      (          µ      ⁢              xe2x80x83            ⁢      m        )  
xe2x80x83and
n: number of particles.
In the laminate polyester film of the invention, a coating layer (B) is formed on one surface of the base layer (A). The coating layer (B) contains a binder resin, added fine particles and a surfactant, and has a thickness of 3 to 40 nm, preferably 4 to 30 nm. The added fine particles contained in the coating layer (B) have an average particle diameter of 10 to 50 nm, preferably 15 to 45 nm, particularly preferably 18 to 40 nm, and the content thereof based on a solid content (a total amount of the binder resin and the surfactant) of the coating layer (B) is in a range of 0.5 to 30% by weight, preferably 2 to 20% by weight, particularly preferably 3 to 15% by weight,
The binder resin in the coating layer (B) includes an aqueous polyester resin, an aqueous acrylic resin and an aqueous polyurethane resin. An aqueous polyester resin is particularly preferred. The aqueous polyester resin will be specifically explained hereinafter.
The above aqueous polyester resin is preferably selected from polyester resins comprising an acid component formed from at least one member selected from polycarboxylic acids such as isophthalic acid, phthalic acid, 1,4-cyclohexanedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4xe2x80x2-diphenyldicarboxylic acid, adipic acid, sebacic acid, dodecanedicarboxylic acid, succinic acid, 5-sodium sulfoisophthalic acid, 2-potassium sulfoterephthalic acid, trimellitic acid, trimesic acid, monopotassium trimellitate and p-hydroxybenzoic acid and a glycol component formed from at least one member selected from polyhydroxy compounds such as ethylene glycol, diethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, 1,4-cyclohexane dimethanol, p-xylylene glycol, dimethylol propane and an adduct of bisphenol A with ethylene oxide. Further, the aqueous polyester resin can be also similarly selected from a graft polymer or block copolymer in which an acrylic polymer chain is bonded to a polyester chain, or an acryl-modified polyester resin in which two types of polymers form a specific physical construction (IPN, core-shell) in a micro molecule.
The above aqueous polyester ester resin may be any type that is soluble, emulsifiable or finely dispersible in water, while a type that is emulsifiable or finely dispersible in water is preferred. Further, the aqueous polyester resin may have a molecule containing, for example, a sulfonic acid salt group, a carboxylic acid salt group, a polyether unit, or the like, which is introduced in the molecule for imparting a hydrophilic nature.
The added fine particles in the coating layer (B) is not critical in type so long as the particles have the above average particle diameter. Preferred are fine particles which hardly precipitate in a coating solution for forming the coating layer and have a relatively low specific gravity. Examples thereof preferably include particles of a heat resistant polymer (such as cross-linked silicone resin, cross-linked acrylic resin, cross-linked polystyrene, melamine-formaldehyde resin, aromatic polyamide resin, polyamide-imide resin, cross-linked polyester, wholly aromatic polyester, or the like), silicon dioxide (silica) and calcium carbonate. Of these, a cross-linked silicone resin, silica and core-shell type organic particles (e.g., particles formed of a cross-linked polystyrene as a core and polymethyl methacrylate as a shell) are particularly preferred.
The surfactant in the coating layer (B) is preferably selected from nonionic surfactants, and particularly preferably selected from surfactants prepared by adding (poly)ethylene oxide to an alkyl alcohol, an alkyl phenyl alcohol or a higher fatty acid.
Examples of the above surfactant includes polyoxyethylene alkylphenylether compounds such as xe2x80x9cNONION NS-230xe2x80x9d, xe2x80x9cNONION NS-240xe2x80x9d, xe2x80x9cNONION HS-220xe2x80x9d and xe2x80x9cNONION HS-240xe2x80x9d which are trade names and supplied by NOF Corporation and xe2x80x9cNONIPOLE 200xe2x80x9d, xe2x80x9cNONIPOLE 400xe2x80x9d, xe2x80x9cNONIPOLE 500xe2x80x9d and xe2x80x9cOCTAPOLE 400xe2x80x9d which are trade names and supplied by Sanyo Chemical Industries Ltd; polyoxyethylene alkylether compounds such as xe2x80x9cNONION E-230xe2x80x9d, xe2x80x9cNONION K-220xe2x80x9d and xe2x80x9cNONION K-230xe2x80x9d which are trade names and supplied by NOF Corporation; and higher fatty acid polyoxyethylene ester compounds such as xe2x80x9cNONION S-15.4xe2x80x9d and xe2x80x9cNONION S-40xe2x80x9d which are trade names and supplied by NOF Corporation.
The amount of the above surfactant based on the solid content of the coating layer (B) is 10 to 50% by weight, more preferably 12 to 40% by weight, particularly preferably 15 to 30% by weight.
When the amount of the surfactant is less than 10% by weight (based on the total solid content), it is difficult to prevent the generation of coarse protrusions which may cause drop-out. When it exceeds 50% by weight (based on a total solid content), a streak-like coating defect occurs due to foaming.
For improving an anti-blocking property, further, the surfactant preferably has a softening point of 30xc2x0 C. or higher when measured according to JIS K7206 (using a dry-solidified surfactant for the measurement). For reducing a surface tension of a coating solution so as to prevent a coating failure in spots during the application of the coating solution for forming a coating layer (B), a surfactant other than the above surfactants may be used in combination so long as the amount thereof does not exceed 10% by weight based on the total solid content.
In the laminate polyester film of the present invention, not only the coating layer (B) is required to have the above-described composition and thickness, but also the surface thereof (surface not in contact with the base layer (A)) is required to satisfy the following characteristics (a) to (d):
(a) the surface has 80,000 pieces/mm2 or less, preferably 60,000 pieces/mm2 or less, of protrusions having a major diameter of 0.05 xcexcm or more when measured on the basis of a surface photograph taken and enlarged at a magnification of 5,000 with a scanning electron microscope,
(b) the surface has 1 to 40 pieces/xcexcm2, preferably 2 to 20 pieces/xcexcm2, of protrusions, when measured on the basis of a surface photograph taken and enlarged at a magnification of 35,000 with a scanning electron microscope,
(c) the surface has 20 pieces/cm2 or less, preferably 10 pieces/cm2, of extra-large protrusions having a major diameter of 10 xcexcm or more, and
(d) the surface has a central plane average roughness (SRa) of 5 nm or less.
The surface characteristics (a) to (d) of the coating layer (B) will be explained further in details.
Of the above surface characteristics of the coating layer (B), the (a) and (c) are characteristics that originate in the influences of the surface characteristics of the base layer (A). That is, the characteristic (a) is a reflection of protrusions of the base layer (A) surface, which protrusions are mainly ascribed to the added fine particles contained in the polyester resin of the base layer (A). The characteristic (c) is a reflection of protrusions of the base layer (A) surface, which protrusions are mainly ascribed to large particles (i.e., particles formed by precipitation of catalysts or stabilizers, or particles included due to other causes) existing in the polyester resin of the base layer (A).
When the surface of the coating layer (B) is measured on the basis of its scanning electron microscopic photograph taken at a magnification of 5,000, the number of protrusions having a major diameter of 0.05 xcexcm or more is 80,000 pieces/mm2 or less, preferably 60,000 pieces/mm2 or less, particularly preferably 50,000 pieces/mm2 or less. The above number depends mainly on the added fine particles contained in the polyester resin of the base layer (A). When the polyester resin of the base layer (A) contains no added fine particles as described above, the above number of the protrusions is extremely small. When the polyester resin of the base layer (A) contains added fine particles, the lower limit of the number of the protrusions is 10,000 pieces/mm2, preferably 20,000 pieces/mm2. When the number of the protrusions exceeds 80,000 pieces/mm2, undesirably, the film is poor in electromagnetic conversion characteristics.
The surface characteristic (b) of the coating layer (B) is explained below. When the surface of the coating layer (B) is measured on the basis of its scanning electron microscopic photograph taken at a magnification of 35,000, the number of protrusions is 1 to 40 pieces/xcexcm2, preferably 2 to 20 pieces/xcexcm2. The number of the above protrusions is more preferably 2.5 to 18 pieces/xcexcm2, particularly preferably 3 to 15 pieces/xcexcm2. The protrusions in the surface characteristic (b) are formed mainly due to the added fine particles contained in the coating layer (B). The number of the above protrusions, therefore, can be brought into the above range by controlling the amount of the added fine particles to be contained in the coating layer (B).
The surface characteristic (c) of the coating layer (B) shows the number of extra-large protrusions. The extra-large protrusions have a major diameter of 10 xcexcm or more, and the number thereof is required to be 20 pieces/cm2 or less and it is preferably 10 pieces/cm2 or less. When the surface has the extra-large protrusions having a height of higher than 100 nm, there is a high possibility of causing drop-out in the resulting product and hence, the number of extra-large protrusions is desirable to be as small as possible. The number of extra-large protrusions having a height of 100 nm or higher should be 10 pieces/cm2 or less, more preferably 5 pieces/cm2 or less.
The extra-large protrusions in the surface characteristic (c) are formed mainly due to a variety of large particles contained in the base layer (A) as described above. That is, the above extra-large protrusions are caused mainly by a contaminant which comes from outside during the film formation, or by residual catalyst particles which precipitate during the polymerization and added fine particles which are poorly dispersed. In most cases, they are caused by the residual catalyst particles and the poorly dispersed added fine particles. When the density of the extra-large protrusions exceeds 20 pieces/cm2, they themselves cause drop-out. When the protrusions are caused by the poorly dispersed added fine particles, undesirably, partial abrasion is liable to be caused on a head, and the electromagnetic conversion characteristics are also caused to be poor.
For decreasing the number of the extra-large protrusions, there is employed a method in which the dispersibility of the added fine particles in the polyester resin is improved, e.g., a method in which a glycol slurry of added fine particles is filtered through a fine filter in advance to exclude large-sized particles including aggregates and the timing of added fine particles being added in the form of a glycol slurry during the polymerization for the polyester is optimized, or a method in which the rate at which the glycol slurry is added is optimized. Or, it is preferred to carry out a high-precision filtration before a molten polymer is extruded through an extrusion die during the film formation. In the high precision filtration, the size of the average mesh openings of a filter, a metal fiber sintered filter in particular, is preferably 50 to 400 times, more preferably 80 to 300 times, the average particle diameter of the added fine particles. The lower limit of the average mesh openings is preferably 1 xcexcm. It is particularly preferred to combine the optimization of the method of adding the particles with the optimization of the average mesh openings in the high precision filtration.
Further, another means for decreasing the number of the extra-large protrusions is to prevent a catalyst or a stabilizer from precipitating as extra-large particles in the production of polyester. For this purpose, the catalyst to be used is selected in kind and amount, and the amount of the stabilizer is controlled. Use of the catalyst and the stabilizer will be described in detail later.
Further, as a surface characteristic (d), the central plane average roughness (SRa) of the coating layer (B) is 5 nm or less, preferably 4 nm or less. When the surface roughness (SRa) exceeds 5 nm, undesirably, the electromagnetic conversion characteristics comes to be low.
The biaxially oriented laminate polyester film of the present invention may have a thin film layer (C) further laminated on a surface of the base layer (A) which surface is not in contact with the coating layer (B). In this case, the laminate polyester film has a three-layered structure of coating layer (B)/base layer (A)/thin film layer (C), and the so-structured laminate film is a preferred embodiment of the present invention.
The above thin film layer (C) may be a coating layer laminated on the surface of the base layer (A) by application, or it may be a layer laminated on the surface of the base layer (A) by co-extrusion in the film formation of the base layer (A). The latter is more preferred.
The thin film layer (C) contains added fine particles. To distinguish the above added fine particles from the added fine particles in the base layer (A) and the coating layer (B) in explanation, the former will be referred to as xe2x80x9cadded fine particles Cxe2x80x9d.
When the thin film layer (C) is a coating layer formed by application, the coating layer can be formed of a binder resin and added fine particles C, and may further contain a surfactant.
The binder resin, the added fine particles C and the surfactant can be selected from those binder resins, added fine particles and surfactants which are explained regarding the coating layer (B).
The average particle diameter of the added fine particles C is preferably 0.01 to 0.1 xcexcm, more preferably 0.02 to 0.08 xcexcm, particularly preferably 0.02 to 0.06 xcexcm. The content of the added fine particles C is preferably 0.5 to 30% by weight, more preferably 1 to 20% by weight, particularly preferably 2 to 10% by weight. In this case, as a surfactant, those surfactants may be used alone or in combination. Therefore, the composition of the thin film layer (C) may be the same as that of the coating layer (B).
Further, the thin film layer (C) may be formed of a polyester resin layer containing the added fine particles C, and may be formed together with the base layer (A) by co-extrusion.
In the thin film layer (C), preferably, the layer thickness thereof, the average particle diameter of the added fine particles C and the content thereof satisfy the following relational expression.
0.001xe2x89xa6(d)3xc3x97cxc3x97txe2x89xa6100
wherein d is an average particle diameter (xcexcm) of the added fine particles in the thin film layer (C), c is a content (% by weight) of the added fine particles, and t is a thickness (nm) of the thin film layer (C).
When the thin film layer (C) is a coating layer formed by application, advantageously, it satisfies the following relational expression.
0.001xe2x89xa6(d)3xc3x97cxc3x97txe2x89xa60.1
When the thin film layer (C) is a layer formed together with the base layer (A) by co-extrusion, advantageously, it satisfies the following relational expression.
0.1xe2x89xa6(d)3xc3x97cxc3x97txe2x89xa6100
In this case, the thin film layer (C) formed by co-extrusion contains the added fine particles C. The added fine particles C have the following average particle diameter and the following content.
The average particle diameter (d) of the added fine particles C is preferably 0.1 to 1 xcexcm, more preferably 0.15 to 0.8 xcexcm, particularly preferably 0.2 to 0.7 xcexcm. The content of the added fine particles C having the above average particle diameter (d) is preferably 0.0001 to 1% by weight, more preferably 0.001 to 0.5% by weight, particularly preferably 0.005 to 0.1% by weight, based on the polyester resin or the binder resin of the thin film layer (C).
Examples of the added fine particles C preferably include (1) heat resistant polymer particles (such as particles of cross-linked silicone resin, cross-linked polystyrene, cross-linked acrylic resin, melamine-formaldehyde resin, aromatic polyamide resin, polyimide resins, polyamide-imide resin or cross-linked polyester), (2) metal oxides (such as aluminum sesquioxide, titanium dioxide, silicon dioxide, magnesium oxide, zinc oxide and zirconium oxide), (3) metal carbonates (such as magnesium carbonate and calcium carbonate), (4) metal sulfates (such as calcium sulfate and barium sulfate), (5) carbon (such as carbon black, graphite and diamond) and (6) clay minerals (such as kaolin, clay and bentonite). Of these, cross-linked silicone resin particles, cross-linked polystyrene particles, melamine-formaldehyde resin particles, polyamide-imide resin particles, aluminum sesquioxide (alumina), titanium dioxide, silicon dioxide, zirconium oxide, synthetic calcium carbonate, barium sulfate, diamond and kaolin are preferred, and cross-linked silicone resin particles, cross-linked polystyrene particles, alumina, titanium dioxide, silicon dioxide and calcium carbonate are particularly preferred.
Further, when the inert fine particles are particles of two or more types, colloidal silica and fine particles of alumina having xcex1, xcex3, xcex4, or xcex8 crystal form are preferably used in combination as second and third particles having an average diameter smaller than the average particle diameter (d) of the added fine particles C. Of particle species shown as the added fine particles C having the average particle diameter (d), fine particles having a small average particle diameter may be used as well.
The average particle diameter of the above fine particles is preferably in the range of from 5 to 400 nm, more preferably 10 to 300 nm, particularly preferably 30 to 250 nm, and is smaller than the above average particle diameter (d) preferably by 50 nm or more, more preferably by 100 nm or more, particularly preferably by 150 nm or more. The content of the second and third particles (fine particles) based on the thin film layer (C) resin is preferably 0.005 to 1% by weight, more preferably 0.01 to 0.7% by weight, particularly preferably 0.05 to 0.5% by weight.
The thickness of the thin film layer (C) is 5 nm or more, preferably 8 nm or more. Desirably, the upper limit of the above thickness is xc2xd or less, preferably ⅓ or less, particularly preferably xc2xc or less, of the total thickness of the laminate polyester film.
The biaxially oriented laminate polyester film of the present invention preferably has an air leak index, measured with the Bekk smoothness tester of Toyo Seiki Co. Ltd, of 0.1 to 2 KPa/hr owing to the presence of the above thin film layer (C).
The biaxially oriented laminate polyester film of the present invention is improved in handling properties and wind-up properties without impairing electromagnetic conversion characteristics, since it has the above thin film layer (C) and exhibits an air leak index in the above value range.
When the thin film layer (C) is laminated by co-extrusion, advantageously, the air leak index is 0.1 to 1.3 KPa/hr, preferably 0.3 to 1 KPa/hr.
In the biaxially oriented laminate polyester film of the present invention, when polyethylene terephthalate is used as a polyester resin for the base layer (A) or the thin film layer (C), the polyethylene terephthalate has an intrinsic viscosity in the range of from 0.4 to 0.9. The intrinsic viscosity is calculated on the basis of a viscosity obtained by measurement of a solution thereof in o-chlorophenol at 35xc2x0 C.
The total thickness of the biaxially oriented laminate polyester film of the present invention is generally 2.5 to 20 xcexcm, preferably 3.0 to 12 xcexcm, more preferably 4.0 to 12 xcexcm.
The biaxially oriented laminate polyester film of the present invention can be produced by a method known per se or according to methods accumulated in the fieled of this art. Of these, a laminate structure of the base layer (A) and the thin film layer (C) is preferably produced by a co-extrusion method, and the coating layer (B) is preferably laminated by an application method.
For example, the case of laminating a base layer (A) and a thin film layer (C) by a co-extrusion method using a polyethylene terephthalate resin as the polyester resin will be explained. A polyethylene terephthalate resin A containing the foregoing added fine particles finely dispersed therein and a polyethylene terephthalate resin C containing the added fine particles C finely dispersed therein are further filtrated through a high-precision filter respectively, and then laminated together in a molten state, within an extrusion die or before an extrusion die (the former is generally called xe2x80x9ca multi-manifold systemxe2x80x9d, and the latter, xe2x80x9ca feed block systemxe2x80x9d) to form a laminate structure having the above proper thickness ratio, and then the laminate structure is co-extruded in the form of a film from the die at a temperature between melting point Tmxc2x0 C. and (Tm+70)xc2x0 C. and quenched on a cooling roll at 40 to 90xc2x0 C. to solidness to obtain an unstretched laminate film. Thereafter, the unstretched laminate film is stretched at a stretch ratio of 2.5 to 8.0, preferably 3.0 to 7.5, in the uniaxial direction (in the longitudinal or transverse direction) at a temperature between (Tgxe2x88x9210) and (Tg+70)xc2x0 C. (Tg: glass transition temperature of the polyethylene terephthalate) and then stretched at a stretch ratio of 2.5 to 8.0, preferably 3.0 to 7.5, in the direction perpendicular to the above direction at a temperature between Tg and (Tg+70)xc2x0 C. according to a conventional method. Further, the laminate film may be re-stretched in the longitudinal direction and/or transverse direction as required. That is, two-stage, three-stage, four-stage or multi-stage stretching may be carried out. The total stretch ratio is generally 9 or more, preferably 12 to 35, more preferably 15 to 30, in terms of an area stretch ratio. Further, the biaxially oriented polyethylene terephthalate film is heat-set and crystallized at a temperature between (Tg+70) and (Tmxe2x88x9210)xc2x0 C., for example, between 180 and 250xc2x0 C., whereby it is imparted with excellent dimensional stability. The time period for the heat-setting is preferably 1 to 60 seconds.
The coating layer (B) is formed by applying a coating solution, preferably aqueous coating solution, containing the above added fine particles, the above binder resin and the above surfactant onto a co-extruded polyester film. Preferably, the coating solution is applied onto the surface of the polyester layer (A) that has not been finally stretched, and then, the coated polyester film is stretched at least in one direction. The coating film is dried before or during the stretching. The application of the coating solution is preferably performed on an unstretched laminate film or longitudinally (uniaxially) stretched laminate film. Particularly preferably, the application is performed on a longitudinally (uniaxially) stretched laminate film. The coating method is not critical, and it includes a roll coating method and a die coating method.
The solid content in the above coating solution, particularly the aqueous coating solution, is preferably 0.2 to 8% by weight, more preferably 0.3 to 6% by weight, particularly preferably 0.5 to 4% by weight. The coating solution (preferably, aqueous coating solution) may contain other components such as other surfactant, stabilizer, dispersant, UV absorber or thickener so long as the effect of the present invention is not impaired.
In the production of the laminate polyester film, additives other than the above added fine particles, such as a stabilizer, a colorant, a resistivity control agent for a molten polymer or the like may be added to the polyester resin, as desired.
In the present invention, for improving various functions as a magnetic recording medium, such as head touch, running durability or the like, and for decreasing the thickness of the film at the same time, the Young""s moduli of the laminate polyester film in the longitudinal direction and in the transverse direction are preferably at least 450 kg/mm2 and at least 600 kg/mm2, more preferably at least 480 kg/mm2 and at least 680 kg/mm2. When the polyester is polyethylene terephthalate, the crystallinity is desirably 30 to 50%. When the crystallinity is lower than the lower limit, the thermal shrinkage factor is large. When it exceeds the upper limit, the abrasion resistance of the film is poor, and white powders are liable to be formed when the film runs being in contact with a roll or a guide pin surface.
A preferred process for producing a polyester resin suitable for forming the base layer (A) in the present invention will be explained below. As described above, in the surface characteristics of the coating layer (B) of the surface of the base layer (A), the number of the extra-large protrusions is required to be extremely few. It is therefore essential to control the formation of large precipitated particles caused by a catalyst or a stabilizer in the polyester resin, which is one of the causes of formation of the extra-large protrusions. The present inventors have therefore made studies and have found that a base layer (A) containing few extra-large protrusions can be obtained by selecting a type and an amount of the catalyst, and bringing the content of the stabilizer into a specific amount ratio, in the polyester resin forming the base layer (A).
That is, preferably, the base layer (A) in the laminate polyester film of the present invention is a film formed of a polyester resin which contains (a) at least one compound selected from the group consisting of an alkali metal compound, an alkaline earth metal compound and a manganese compound, (b) a phosphorus compound and (c) at least one compound selected from the group consisting of a germanium compound, a titanium compound and an antimony compound in an amount ratio satisfying the following expressions (1) to (6) simultaneously:
25xe2x89xa6Mxe2x89xa6250xe2x80x83xe2x80x83(1)
0.1xe2x89xa6(M/P)xe2x89xa66.0xe2x80x83xe2x80x83(2)
0xe2x89xa6Tixe2x89xa612.5xe2x80x83xe2x80x83(3)
0xe2x89xa6Gexe2x89xa6110xe2x80x83xe2x80x83(4)
0xe2x89xa6Sbxe2x89xa650xe2x80x83xe2x80x83(5)
0.3 less than (Ti+Ge)xe2x89xa6110xe2x80x83xe2x80x83(6),
wherein M is a content (ppm) of a total of an alkali metal element, an alkaline earth metal element and a manganese element from the compound (a), P is a content (ppm) of a phosphorus element, Ti is a content (ppm) of a titanium element, Ge is a content (ppm) of a germanium element and Sb is a content (ppm) of an antimony element, in the polyester.
The alkali metal element, alkaline earth metal element, manganese element, phosphorus element, titanium element, antimony element and germanium element exist alone or as complex compounds in the polyester. It is therefore required to optimize the contents and the ratio of these elements of alkali metal, alkaline earth metal, manganese, phosphorus, titanium, antimony and germanium.
Preferably, the compounds of the above elements have functions capable of improving the activity of a reaction catalyst and stabilization properties during the polyester production or electrostatic adhesion properties during the film production. For example, the alkali metal compound and the alkaline earth metal compound are preferably selected from oxide, chloride, carbonate, carboxylate or acetate of a lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium or barium element. Particularly preferred is an acetate of lithium, sodium, potassium, cesium, magnesium, calcium or barium element.
The manganese compound is preferably selected from oxide, chloride, carbonate or carboxylate, and an acetate is particularly preferred.
Further, the phosphorus compound is preferably selected, for example, from phosphoric acid, a phosphate ester such as trimethyl phosphate, triethyl phosphate, triphenyl phosphate or acidic methyl phosphate ester, phosphorous acid, a phosphate ester such as trimethyl phosphate, methylphosphonic acid, phenylphosphonic acid, benzylphosphonic acid, a phosphonate ester such as methyl methylphosphonate, ethyl phenylphosphonate or phenyl benzylphosphonate, trimethyl phosphate, triethyl phosphate, or tri-n-butyl phosphate.
The titanium compound is preferably selected, for example, from titanium tetrabutoxide, titanium trimellitate, tetraethoxytitanium, titanium sulfate or titanium chloride.
The germanium compound is preferably selected, for example, from amorphous germanium dioxide, hexagonal germanium dioxide, germanium chloride or germanium phosphate.
The antimony compound is selected, for example, from antimony trioxide, antimony pentoxide, antimony acetate, antimony potassium tartrate, antimony oxychloride or triphenyl antimony.
The above compounds are preferably soluble in glycol, and can be used alone or in combination of two or more.
The total content (M) of an alkali metal element, an alkaline earth metal element and a manganese element in the polyester is required to be in the range of from 25 to 250 ppm, and it is preferably in the range of from 50 to 200 ppm (the above requirement (1)). When the above content is less than 250 ppm, for example, the ester interchange reaction is deferred in the production of polyester by an ester interchange method, and a film is poor in electrostatic adhesion properties, which decreases the productivity of the film. When the above content exceeds 250 ppm, precipitated particles are formed due to compounds of elements contained, and the surface flatness of the formed film is poor.
Further, in the present invention, the ratio (M/P) (a ratio of values of a ppm unit) of the total content (M) of an alkali metal element, an alkaline earth metal element and a manganese element to the content of a phosphorus element (P) in the polyester is required to be in the range of from 0.1 to 6.0, and it is preferably in the range of 0.5 to 4.5, more preferably in the range of 0.5 to 2.0 (the above requirement (2)). When the above ratio is less than 0.1 or when it is more than 6.0, precipitated particles are formed due to compounds of elements contained, resulting in a poor surface flatness of a formed film.
The polyester is required to contain a titanium compound, a germanium compound or an antimony compound as a polycondensation catalyst. The content of the titanium compound is required to be 12.5 ppm or less, and it is preferably 7 ppm or less. When the above amount is more than 12.5 ppm, undesirably, the durability against deterioration under dry heat is low during the film formation.
The content of the germanium element is required to be 110 ppm or less, and it is preferably 80 ppm or less. When the above amount exceeds 110 ppm, the durability against deterioration under dry heat is low during the film formation.
Further, the content of the antimony element is required to be 50 ppm or less, and it is preferably 25 ppm or less. When the above amount is more than 50 ppm, black foreign matter is liable to be formed due to the antimony. Particularly when polyethylene terephthalate is used, undesirably, foreign matters come to noticeably increase with the passage of time during the film formation (the above requirements (3) to (5)).
Furthermore, of the above catalysts as a polycondensation catalyst, it is preferred to use the titanium compound and the germanium compound in a total amount of 0.3 to 110 ppm, preferably 1 to 80 ppm (the above requirement (6)).
In the production of the polyester, The timing and the method of adding the alkali metal compound, the alkaline earth metal compound, the manganese compound, the phosphorus compound, the titanium compound, the germanium compound and the antimony compound are not critical. For example, it is preferred to add the alkali metal compound, the alkaline earth metal compound and/or the manganese compound until the intrinsic viscosity of a reaction product reaches 0.2. In the ester interchange method, it is preferred to add the compound(s) before initiation of the ester interchange reaction, since it (they) can be used as an ester interchange catalyst. The above reaction may be carried out at atmospheric pressure or under elevated pressure, and it is preferred to carry out the reaction under elevated pressure, since the reaction time period can be decreased. Concerning the order of addition, the compounds may be added simultaneously or separately.
For example, in the ester interchange method, the phosphorus compound is added preferably after substantial completion of the esterification. The phosphorus compound may be added all at once, or may be divided for adding twice or more.
Further, it is preferred to add the titanium compound or the germanium compound at any time that is 10 minutes or more after the addition of the phosphorus compound but is before the intrinsic viscosity reaches 0.3.
Further, the antimony compound may be added at any time before the intrinsic viscosity reaches 0.3.
Since the phosphorus compound deactivates the polymerization catalyst activity of the titanium compound, the reaction time period for the polycondensation can be decreased by adding the phosphorus compound at a last stage of the polycondensation. Further, for example, at least two polymers can be blended in a molten state in an extruder for the film formation. It is also preferred to prepare a master polymer having an increased content of the phosphorus compound and to blend it with a polymer containing the titanium compound.
Further, the polyester may contain other soluble metal component, e.g., a compound containing an Zn element or a Co element, so long as the effect of the present invention is not impaired.
As described above, the number of the extra-large protrusions in the base layer (A) can be controlled by using the catalysts and stabilizers in combination so as to satisfy the requirements (1) to (6). Further, according to the studies made by the present inventors, it has been found that the number of the extra-large protrusions in the base layer (A) can be decreased by another combination different from the above combination.
That is, in the laminate polyester film of the present invention, the base layer (A) is preferably a film formed of a polyester resin which contains a magnesium compound, a calcium compound, a titanium compound and a phosphorus compound in an amount ratio satisfying the following expressions (7) to (10) simultaneously.
25xe2x89xa6(Mg+Ca)xe2x89xa6200xe2x80x83xe2x80x83(7)
0.3xe2x89xa6Mg/Caxe2x89xa610xe2x80x83xe2x80x83(8)
0.5xe2x89xa6(Mg+Ca)/Pxe2x89xa66.0xe2x80x83xe2x80x83(9)
0.3xe2x89xa6Tixe2x89xa612.5xe2x80x83xe2x80x83(10),
wherein Mg, Ca, Ti and P show contents (ppm) of a magnesium element, a calcium element, a titanium element and a phosphorus element in the polyester, respectively.
In the above polyester, the calcium compound and the magnesium compound used for its production are preferably selected from oxides, chlorides, carbonates or carboxylates of these compounds, and acetates such as calcium acetate and magnesium acetate are particularly preferred.
These calcium and magnesium compounds may be added at any stage of the polyester production steps.
The above calcium compound and the above magnesium compound have an effect of decreasing the specific resistance, for example, of a molten polyalkylene naphthalate. They can be also used as an ester interchange reaction catalyst when an ester interchange reaction of dimethyl naphthalenedicarboxylate and aliphatic glycol is employed in the stage of polymer production. For this reason, when these compounds are used as the ester interchange reaction catalysts, they are added before initiation of the ester interchange reaction.
When it is not required to work the calcium compound and the magnesium compound as a catalyst, the timing of adding them is not specially limited. In an preferred embodiment, they are added before the intrinsic viscosity of the reaction product reaches 0.2, since it is easy to disperse these compounds homogeneously in the polymer. In this case, they may be added together at the same time, or they may be added separately one in one time and the other in another time. The esterification can be carried out in the absence of a catalyst.
The above calcium compound and the above magnesium compound are used in such an amount that the total content of calcium and magnesium elements in the polyester is 25 to 200 ppm, preferably 50 to 180 ppm, particularly preferably 70 to 160 ppm (requirement (7)). For example, when the polymer is produced by an ester interchange method, the magnesium compound and the calcium compound are added as ester interchange catalysts in such an amount that they are soluble in the reaction system. When the above amount exceeds 200 ppm, undesirably, the surface flatness of a formed film is affected by an influence of precipitated particles caused by the catalysts. When the content is less than 25 ppm, undesirably, not only the ester interchange in the production of the polyester by an ester interchange method proceeds insufficiently, but also the subsequent polymerization proceeds slowly.
The amount ratio (Mg/Ca) of magnesium to calcium by the unit of ppm is 0.3 to 10, preferably 0.5 to 9, particularly preferably 2.0 to 8.0 (requirement (8)). When the above ratio is less than 0.3, the surface flatness of a formed film is affected by the influence of precipitated particles caused by the catalysts. When the ratio exceeds 10, undesirably, a formed film is poor in various characteristics.
Further, the ratio of the total amount of magnesium and calcium to the amount of a phosphorus compound by the unit of ppm is 0.5 to 6.0, preferably 1.0 to 5.0, particularly preferably 2.0 to 5.0 (requirement (9)). When the above ratio by the unit of ppm exceeds 6, undesirably, the surface flatness of a formed film is affected by an influence of precipitated particles caused by the catalysts. When the above ratio is less than 0.5, undesirably, the phosphorus compound is present in an excess amount relative to the ester interchange catalysts, so that the polycondensation proceeds slowly.
The phosphorus compounds to be used are selected from those compounds described with regard to the above polyester.
The phosphorus compound can be added before the polyester is formed into a film. For example, it may be added after completion of the esterification or the ester interchange reaction, or it may be added just before the polycondensation. The phoshprus compound may be added all at once, or may be divided into two or more portions for adding them. Further, at least two polymers can be blended in a molten state in an extruder for the film formation. It is also preferred to employ a method, for example, in which a master polymer having an increased content of the phosphorus compound is prepared and then, blended with a polymer containing the titanium compound and the inert fine particles to obtain a polymer (composition) which satisfies the above requirements (7) to (10).
Although not specially limited, the titanium compound for use preferably includes, for example, titanium tetrabutoxide, titanium trimellitate, tetraethoxytitanium, titanium sulfate and titanium chloride. Of these, titanium tetrabutoxide and titanium trimellitate are particularly preferred.
In the ester interchange method, the above titanium compound may be added before initiation of the ester interchange reaction, during the ester interchange reaction, after completion of the ester interchange reaction or just before the polycondensation reaction. In the esterification method, further, it may added after completion of the esterification or just before the polycondensation.
The content of the titanium compound as a titanium element in the polyester is 0.3 to 12.5 ppm, preferably 1 to 10 ppm. When the above content exceeds 12.5 ppm, the film is liable to deteriorate under dry heat due to the titanium. When it is less than 0.3 ppm, undesirably, the polymerization proceeds slowly.
According to the present invention, there is also provided a magnetic recording medium having the above biaxially oriented laminate polyester film as a base film, i.e., a magnetic recording medium comprising the above biaxially oriented polyester film and a magnetic layer present on the coating layer (B) of the above laminate film.
A working embodiment of production of the magnetic recording medium of the present invention is as follows.
A ferromagnetic metal thin film layer of iron, cobalt, chromium or an alloy or oxide composed mainly of these is formed on the surface on the coating layer (B) of the above biaxially oriented laminate polyester film by a vacuum vapor deposition, sputtering or ion plating method, a protective layer of diamond-like carbon (DLC) or the like, and a fluorine-containing carboxylic acid lubricant layer are consecutively formed on the surface thereof depending upon a purpose, use or necessity, and further a back coating layer is formed on the surface on the thin film layer (C) side by a known method as required, whereby there can be produced a high-density-recording deposited magnetic recording medium which is excellent in output in a short-wavelength region and electromagnetic conversion characteristics such as S/N and C/N and of which the drop-out and the error rate are decreased. The above deposited magnetic recording medium is very useful as a tape medium for an Hi8 for analog signal recording and a digital video cassette recorder (DVC), 8 mm data recorder and DDSIV for digital signal recording.
A needle-shaped fine magnetic powder (metal powder) formed of iron or formed of iron as a main component is homogeneously dispersed in a binder such as polyvinyl chloride or a vinyl chloride-vinyl acetate copolymer, the dispersion is applied onto the surface of the coating layer (B) of the above biaxially oriented laminate polyester film so as to form a magnetic layer having a thickness of 1 xcexcm or less, preferably 0.1 to 1 xcexcm, and a back coat layer is formed on the surface on the thin film layer (C) side by a known method as required, whereby there can be produced a high-density-recording metal coated magnetic recording medium which is excellent in output in a short wavelength region and electromagnetic conversion characteristics such as S/N and C/N and of which the drop-out and the error rate are decreased. Further, as required, there may be employed a constitution in which fine titanium oxide particles are dispersed in the same organic binder as that used for forming the magnetic layer and the dispersion is applied onto the base layer (A) to form a non-magnetic layer as an undercoating layer for the above metal powder-containing magnetic layer. The above metal coated magnetic recording medium is very useful as a magnetic tape medium for 8 mm video, Hi8, xcex2-cam SP and W-VHS for analog signal recording and digital video cassette recorder (DVC), 8 mm data recorder, DDSIV, digital xcex2-cam, D2, D3 and SX for digital signal recording.
Further, a needle-shaped fine magnetic powder of iron oxide or chromium oxide or a sheet-like fine magnetic powder such as barium ferrite is homogeneously dispersed in a binder such as polyvinyl chloride or vinyl chloride-vinyl acetate copolymer, the dispersion is applied onto the surface of the coating layer (B) of the above biaxially oriented laminate polyester film to form a magnetic layer having a thickness of 1 xcexcm or less, preferably 0.1 to 1 xcexcm, and a back coat layer is formed on the surface on the thin film layer (C) side by a known method as required, whereby there can be produced a high-density-recording oxide coated magnetic recording medium which is excellent in output in a short wavelength region and electromagnetic conversion characteristics such as S/N and C/N and of which the drop-out and the error rate are decreased. Further, as required, there may be employed a constitution in which fine titanium oxide particles are dispersed in the same organic binder as that used for forming the magnetic layer and the dispersion is applied onto the thin film layer (C) to form a non-magnetic layer as an undercoating layer for the above magnetic-powder-containing magnetic layer. The above oxide coated agnetic recording medium is useful as a high-density oxide coated magnetic recording medium such as QIC for a data streamer of digital signal recording.
The above W-VHS is a VTR for analog HDTV signal recording, and the above DVC is applicable to digital HDTV signal recording. It can be said that the biaxially oriented laminate polyester film of the present invention is a base film remarkably useful for a magnetic recording medium for the VTRs applicable to HDTV signals.